Microporous films formed from polymer materials have been used for various purposes, for example, medical or industrial filtration membranes or separation membranes and separators such as battery separators and condenser separators.
In recent years, as a demand for secondary batteries as one power source for cellular phones, mobile computers and automobiles has increased, and a demand for battery separators has increased. With the spread of the separators in automobile batteries among them, densification of energy has been required, and ensuring of safety of batteries has been needed more and more. In order to ensure safety, it is essential for a separator to have shutdown function, but if the temperature further rises after shutdown to thereby melt the whole of the separator and to cause film breakage, electrical insulation cannot be maintained. On this account, improvement in heat resistance of separators has been desired. However, the melting point of ultrahigh-molecular weight polyethylene that is a main material of separators is as low as about 140° C., and the heat resistance is limited. Then, polypropylene having a high melting point has been used for microporous films of high heat resistance. For microporous films, piercing strength to prevent damage done when the film strength is low and external force is applied is generally also desired.
As production processes for microporous films of polypropylene, there are a wet process and a dry process. The wet process is a process comprising forming a film from a resin composition obtained by blending polypropylene with a filler and a plasticizer and extracting the filler and the plasticizer from the film to produce a microporous film. This process has not only a working environmental problem in the extraction step and an environmental problem such as disposal of an extraction liquid but also a possibility of remaining of small amounts of the filler, the plasticizer and the extraction liquid in the microporous film, so that there are fears for performance and safety of a battery. On the other hand, the dry process is a process comprising preparing a raw fabric film of polypropylene and then subjecting the film to cold stretching and hot stretching to form micropores in the film. Since this process does not use a filler, a plasticizer and an extraction liquid at all, such a working environmental problem and such an environmental problem of disposal of the extraction liquid as above do not exist. Further, since a filler, a plasticizer and an extraction liquid are not used, this process is a production process of low cost. Furthermore, there is no possibility of remaining of small amounts of a filler, etc. in the separator, and performance and safety of the battery are not impaired. However, the dry process is a production process in which micropores are formed by stretching, so that permeability and strength of the microporous film are greatly influenced by the properties of polypropylene and the film-forming and stretching conditions. On this account, problems of the microporous film are a narrow range of conditions for forming the microporous film, processing stability and quality stability of the film.
In a patent literature 1, a porous film for a battery separator, which has a high porosity, a large maximum pore diameter and a high permeability and has excellent lithium ion conduction property, and a production process for the film have been disclosed. However, since this porous film has a high molecular weight, the discharge quantity in the raw fabric film production is small, and the productivity is poor.
In a patent literature 2, a fine porous membrane of highly crystalline polypropylene, which has a uniform pore size distribution, a high pore density and a high porosity, has been disclosed. However, improvement in the ranges of membrane-forming conditions and stretching conditions for the fine porous membrane has not been studied at all.
In a patent literature 3, a microporous membrane-forming propylene polymer having excellent heat resistance and strength has been disclosed. However, the microporous film has poor permeability. In a patent literature 4, a microporous film obtained by the use of a polypropylene resin composition having a specific elongational viscosity and a specific shear viscosity has been disclosed, and a film having an excellent permeability has been disclosed. However, polypropylene having an excellent balance between film strength and permeability has not been studied at all.
In patent literatures 5, 6 and 7, a microporous film having excellent lithium ion permeability has been proposed, but a film having an excellent balance between film strength and permeability has not been studied at all, similarly to the patent literature 4.
Also in patent literatures 8, 9, 10 and 11, a microporous film having excellent lithium ion permeability has been proposed. Further, film strength has been also referred to, but there is merely description of tensile yield strength in the Example. In general, the tensile yield strength is a stress measured when the film is in the non-broken state, and it is well known that this has no relation to breaking strength of a film, such as piercing strength. Therefore, improvement in breaking strength of the porous film has not been substantially studied at all. Moreover, the range of the molding conditions for the microporous film is not studied either.